According to studies at hand, around 19% of Brazil is rh negative. How can this be?
Is it similar to how things are in Chile where parts are almost zero percent rh negative while others more than 20%?
To get an idea of why, we must examine ancestry. Native Americans were 0% rh negative, so this is not where the rh negative frequency would come from. So what are the mtDNA and y-DNA frequencies among Brazilians?
So let’s look at the first clue: Y-DNA
But mtDNA is completely different showing more native and African haplogroups. Here is one example from one of the studies highlighted below:
In the state of Piauí, individuals descending from Amerindian maternal lineages accounted for 52.3%, followed by African, European, and non-Amerindian Asian, each with 36.2%, 9.8% and 1.7%, respectively. In Ceará, Amerindian lineages were present in 51.9% of the samples, followed by 30.8% African, 11.5% European, and 5.8% non-Amerindian Asian. Similarly, in Rio Grande do Norte 45.7% of the population has Amerindian ancestry, while 35.1% as African descendants, 15.6% are European and 3.6% are non-Amerindian Asian.
We are told, that Europeans brought along diseases wiping out much of the native population. But that cannot account for the huge difference between indigenous y-DNA and mtDNA present today.
Did those diseases only kill the native men and not the women?
But how does this relate to the rh negative blood factor?
Here is the second clue:
More than 50% of the Y chromosomes belong to the R1 branch, namely to the sub-lineage R1b1a-M269, which presents high frequencies throughout Western Europe.
R1b1a-M269 is the most frequent haplogroup in Europe, presenting a cline distribution with high frequencies in the West, decreasing towards the East. The highest frequencies of this haplogroup were reported for populations in Ireland. Its prevalence is also high across the Iberian Peninsula, especially in populations from Basque Country and the Pyrenees.
Whether or not R1b is where rh negative blood originates is the question. It could have been that the Proto-Celts gotten it from the Proto-Basques they’ve once invaded. The original Basque male population has almost completely been replaced a few thousand years ago creating a population bottleneck. But generally speaking nowadays, regions high in y-DNA R1b are also high in rh negative blood. Except the Welsh whose frequencies appear to be similar to those of most of Europe.
In addition, more than 5 Million Europeans have immigrated to Brazil since the early 1,800s. And many of them were Iberian as well.
Conclusion:
Since around 90% of the y-DNA in Brazil is of European ancestry and around 90% of the mtDNA is non-European, it is fair to say that around half of the gene pool in Brazil is European. Potentially, half of that European DNA is of Basque origin. This indicates that the study showing 19% rh negative frequencies in Brazil could be genuine on a national level. Even though it is quite likely, that there are regions where rh negative percentages are significantly lower. More studies are needed to determine how in Brazil rh negative blood type frequencies vary by region and what an accurate national frequency is based on such data.
Regarding y-DNA, see:
Male Lineages in Brazil: Intercontinental Admixture and Stratification of the European Background
Regarding mtDNA, see:
The Ancestry of Brazilian mtDNA Lineages
mtDNA structure: the women who formed the Brazilian Northeast